This invention relates to a terminal table unit that serves to receive input data from an external input device such as a proximity switch for relaying between a control device such as a controllable programmer (PLC) and an external output device such as a solenoid for monitoring this external output device and further to control the external output device on the basis of such input data and in response to a command from the control device.
There is a demand for reducing the down-time as much as possible when a production line, for example, is stopped due to an abnormal condition. When such an abnormal condition occurs, say, in a production line, the usual routine is for a maintenance manager on the site to investigate the parts considered to be causing the abnormal condition and to replace or to repair components at the parts considered to be in an abnormal condition. If the production line operates correctly after such replacement or repair, the routine is ended as having been completed.
FIG. 12 shows a prior art remote monitoring system 43 described in Japanese Patent Publication Tokkai 2002-73432, adapted to monitor a target apparatus 41 by using a data collecting unit 40 to collect detection signals outputted from the target apparatus 41 and supplying the collected detection signals to a monitoring unit 42 while the monitoring unit 42 is normally functioning. If the monitoring unit 42 is not functioning normally, the collected detection signals are temporarily saved and then supplied to the monitoring unit 42 when the latter returns to the normal condition so as to restart the monitoring of the target apparatus 41. In FIG. 12, numeral 44 indicates a data processor, numeral 45 indicates a CRT monitoring device, numeral 46 indicates a message printer, numeral 47 is a gateway device, numeral 48 indicates an I/O controller and numeral 49 indicates a memory.
If a user repairs or replaces a component believed to have been the cause of an abnormal condition and the production line begins to operate normally thereafter, the work is believed to have been completed but there are situations where the component which has been repaired or replaced was not the major cause of the abnormal condition although the production line happens to start operating seemingly in a normal fashion. In such a situation, since the real cause of the abnormal condition has not been removed, the same trouble is likely to recur shortly thereafter, say, again only a few months later.
From the point of view of safety, it is desirable to be able to recognize a sign or a symptom of an on-coming abnormal condition but it is difficult to bring back a stopped condition due to an abnormality. Since the sequence of events immediately before such an abnormal condition cannot usually be replayed, it is not possible to analyze them to take any appropriate measures.
It may be proposed to form a network system including units for input and output such that data can be exchanged among them but such a method has its own problem that once such a network has been formed, the structure of the network itself as well as its wiring will have to be changed whenever a device is changed and hence the loss may become significantly large.